The present invention relates to a horizontal deflection circuit for a CRT (cathode ray tube), used for example in a television receiver, which enables the horizontal sweep width of CRT scanning to be freely adjusted.
FIG. 1 shows a circuit diagram of an example of such a prior art horizontal deflection circuit, having a configuration whereby adjustment can be performed of the level of horizontal deflection current which flows in a horizontal deflection coil of the CRT. In the following description of prior art examples and of embodiments of the present invention, it will be assumed for convenience of description that the horizontal deflection circuit is utilized in a television receiver, however it should be understood that the present invention is equally applicable to various other types of apparatus which employ raster scanning of a CRT.
Reference numeral 1 denotes a horizontal output transistor which receives excitation pulses P from a preceding circuit stage (not shown in the drawings), to be periodically switched between a conducting and a non-conducting state. The first half of the saw-tooth wave scanning current is mainly supplied from a damper diode 2. Numeral 3 denotes a flyback resonance capacitor, numeral 4 a horizontal deflection coil which is connected at one end thereof to the collector of horizontal output transistor 1, numeral 5 a sweep width adjustment coil which is connected at one end thereof to the other end of the horizontal deflection coil 4, and numeral 6 an S-curve correction capacitor which is connected between ground potential and the other end of sweep width adjustment coil 5. A flyback transformer 7 is provided with a primary winding 7a, a secondary winding 7b, and a tertiary winding 7c. A DC potential E.sub.b of a power source for the horizontal deflection circuit is applied to one end of the primary winding 7a of flyback transformer 7, while the other end of primary winding 7a is connected to junction of the collector of horizontal output transistor 1 and horizontal deflection coil 4. The switching operation of output transistor 1 results in flyback pulses being produced across the primary winding 7a of flyback transformer 7. The secondary winding 7b of flyback transformer 7 performs voltage step-up of these flyback pulses, and the stepped-up pulses are applied to a high-voltage DC generating circuit 8 (i.e. a high-voltage rectifier circuit) for generating an EHT voltage to be applied to the anode of the CRT. In addition, pulses designated in FIG. 1 as P.sub.o are produced from the tertiary winding 7c of flyback transformer 7. These may be reflected and smoothed to provide a DC power source for other circuits of the television receiver.
The operation of a horizontal deflection circuit of the form shown in FIG. 1 is well known, and the circuit produces a current flow (designated in the following as I.sub.y) having a sawtooth waveform in the horizontal deflection coil 4, to thereby execute horizontal sweep deflection of the electron beam of the CRT. The peak-to-peak value of this sawtooth waveform current I.sub.y is given as follows, designating the power supply voltage applied to the horizontal deflection circuit as E.sub.b, the horizintal scanning period as T.sub.s, the impedance of the horizontal deflection coil 4 as L.sub.y, the impedance of the sweep width adjustment coil 5 as L.sub.a : EQU I.sub.y =E.sub.b .multidot.T.sub.s /(L.sub.y +L.sub.a) (1)
If the impedance L.sub.a of the sweep width adjustment coil 5 is increased then the amplitude of the sawtooth waveform current I.sub.y which flows in the horizontal deflection coil 4 will be reduced, and conversely a reduction of the impedance L.sub.a will produce an increase in the amplitude of current I.sub.y. In this way, the amplitude of the sawtooth waveform current I.sub.y, and hence the horizontal sweep width of the CRT raster, can be adjusted as required by varying the impedance of the sweep width adjustment coil. This impedance will be designated in the following as L.sub.a. With the circuit arrangement shown in FIG. 1, adjustment of the horizontal sweep width is performed by direct variation of impedance L.sub.a of the sweep width adjustment coil 5. However it is also possible to perform such adjustment by utilizing a saturable reactor. An example of this is shown in FIG. 2. Instead of the sweep width adjustment coil 5 used in the circuit of FIG. 1, the controlled windings 9a, 9a' of a saturable reactor 9 are connected in series with the horizontal deflection coil 4. A source of a current Idc is connected to the control winding 9b of saturable reactor 9, and the value of this current Idc is varied to thereby alter the inductance of controlled windings 9a and 9a'.
In the example of FIG. 2, the source of current Idc is an amplifier 10, which produces an output current that is determined by an input voltage applied to amplifier 10 from the slider of a potentiometer 11. The potentiometer 11, amplifier 10 and primary winding 9b are respectively coupled to a source of a supply voltage E. In this way, the level of current Idc which flows in the control winding 9b can be varied by adjustment of the potentiometer 11, whereby the inductance of the controlled windings 9a and 9a' of saturable reactor 9 can be adjusted.
The saturable reactor 9 can have, for example, the configuration shown in FIG. 3. Each of the arms of the reactor is formed of an E-E type core or an E-I type of core. The controlled windings 9a and 9a' are respectively wound on the two side arms of the reactor core in the directions shown in FIG. 3, while the control winding 9b is wound around the central arm of the core. When a DC current Idc is passed through the control winding 9b then a corresponding magnetic flux .phi.b is produced, and when a current I.sub.y is passed through the controlled windings 9a and 9a' (i.e. around the two side arms of the saturable reactor), magnetic fluxes .phi.a and .phi.a' will be respectively produced in these side arms. The direction of the magnetic flux .phi.b will be identical to that of one of the fluxes .phi.a or .phi.a'. As a result, if the levels of current flow are sufficiently high, the one of the two side arms in which this flux direction coincidence occurs will become magnetically saturated, so that the sum of the inductance values of the ontrolled windings 9a and 9a' (designated in the following as L.sub.s) will be reduced. Thus, the value of L.sub.s can be varied as required, by varying the level of DC current Idc which flows in the control winding.
FIG. 4 illustrates the relationship between variation of DC current Idc and the inductance value L.sub.s. As is clear from FIG. 4, as the level of Idc is increased, the value of L.sub.s is reduced. With the circuit shown in FIG. 2, the horizontal deflection current I.sub.y which flows in horizontal deflection coil 4 (connected in series with controlled windings 9a and 9a' of saturable reactor 9) will be increased by such a reduction of inductance value L.sub.s, and the horizontal sweep width will thereby be increased. Similarly, the horizontal sweep width can be reduced as required, by reducing the level of Idc, to thereby increase the value of inductance L.sub.s.
However certain disadvantages arise with the use of such a saturable reactor for adjustment of horizontal sweep width. The inductance value L.sub.s, which is the sum of the inductance values of controlled windings 9a and 9a' of the saturable reactor 9, is affected by the amplitude of the horizontal deflection current I.sub.y which flows through these controlled windings 9a and 9a'. Thus as shown in FIG. 5, the inductance value L.sub.s is increased when the horizontal deflection current I.sub.y is close to zero (i.e. in the region of the center of the horizontal scanning range), and decrease as I.sub.y increases. As a result, for example if a video signal representing a lattice is to be displayed on the CRT, then the distance between successive lines of the lattice will be reduced, within that part of the display which is situated close to the center of the CRT screen. This effect is illustrated in FIG. 6. It can thus be understood that with this prior art circuit, it is not possible to attain satisfactory adjustment of the horizontal sweep width.
Furthermore, with the horizontal deflection circuits shown in FIGS. 1 and 2 respectively, switching operation is performed in response to the supply of excitation pulses P, whereby pulses P.sub.c having a substantially sinusoidal waveform are produced at the collector of the horizontal output transistor 1. In the case of the circuit arrangement of FIG. 1, if the value of the capacitance of the flyback resonance capacitor 3 is designated as C, and the sum of the inductance of the flyback transformer 7 as seen from the primary winding 7a, the inductance of the horizontal deflection coil 4, and the inductance of the sweep width adjustment coil is designated as L, then the peak value of the pulses P.sub.c is expressed by equation (2) given below. Also, in the case of the circuit arrangement of FIG. 2, if the value of the capacitance of the flyback resonance capacitor 3 is designated as C, the sum of the inductance of the flyback transformer 7 as seen from the primary winding 7a, the inductance of the horizontal deflection coil 4, and the sum of the inductance values of the controlled windings 9a and 9a' of saturable reactor 9 is designated as L, then the peak value of the pulses P.sub.c is expressed by equation (2). EQU P.sub.c =E.sub.b .multidot.T.sub.s /(2Tr)+E.sub.b ( 2)
In equation (2), Tr denotes the horizontal retrace time. The value of this is given as : EQU =Tr.pi..sqroot.L.C (3)
Thus, with the current of FIG. 1, if for example the inductance value L.sub.a of the sweep width adjustment coil is reduced, (or with the circuit of FIG. 2, if the inductance value L.sub.s which is the sum of the inductances of controlled windings 9a and 9a' of saturable reactor 9 is reduced), then from equation (1) above the value of the sawtooth waveform current I.sub.y which flows through the horizontal deflection coil 4 will be increased. Thus the horizontal sweep width will be accordingly increased. However in addition to this if the inductance L.sub.a of the sweep width adjustment coil 5 is reduced, or if the value of L.sub.s of controlled windings 9a and 9a' is reduced, then from equation (3) above it can be understood that the composite inductance value L will be reduced. Hence, from equation (3), it can be seen that the value of the horizontal retrace time Tr will be reduced, and the amplitude of the collector pulses P.sub.c which are produced at the collector of horizontal output transistor 1 will be increased.
The amplitude of the collector pulses P.sub.c applied to the primary winding 7a of flyback transformer 7 is stepped-up by the secondary winding 7b, and the stepped-up pulses are rectified in high-voltage DC generating circuit 8 to provide the EHT anode voltage for the television receiver CRT. Thus, such an increase in the amplitude of collector pulses P.sub.c will result in a corresponding increase in the amplitude of the stepped-up pulses produced from secondary winding 7b of flyback transformer 7, and hence an increase in the value of the EHT voltage which is applied from circuit 8 to the anode of the CRT.
However, as is well known, an increase in the anode EHT voltage of the CRT will result in a lowering of the deflection efficiency of the CRT. Thus, if adjustment is performed to reduce the horizontal sweep width by adjustment of the inductance of the sweep width adjustment coil 5 in the case of the circuit of FIG. 1, (or by adjustment of the inductance L.sub.s of controlled windings 9a and 9a' of saturable reactor 9 in the case of the circuit of FIG. 2), the effect of this adjustment will be counteracted by a resultant increase in the anode EHT voltage. A similar result will occur when it is attempted to increase the horizontal sweep width by adjustment of the inductance of sweep width adjustment coil 5 (or adjustment of L.sub.s of controlled windings 9a and 9a').
Furthermore, the variation in amplitude of the collector pulses P.sub.c resulting when adjustment is performed of the inductance of the sweep width adjustment coil 5 in the circuit of FIG. 1 (or when the total inductance value L.sub.s of the controlled windings 9a and 9a' is adjusted, in the circuit of FIG. 2) to alter the horizontal sweep width will produce a variation in the amplitude of the pulses P.sub.o which are produced from the tertiary winding 7c of flyback transformer 7. If these pulses P.sub.o are used to produce a supply voltage for other circuits of the television receiver, then problems will result from such variations in the amplitude of the pulses.